1. Field of the Invention
The invention relates to movable platform devices, particularly the type in which a movable platform or stage is supported on a stationary member or base. In particular, the invention provides a stiffener which prevents undesirable movement of the stage with respect to the base. Although the present invention is particularly advantageous in extremely high precision stage assemblies, various aspects of the present invention are also applicable to a wide variety of applications in which a controlled sliding movement is utilized.
2. Discussion of the Background
Assemblies in which sliding movement is provided between a first movable member and a second stationary member are well known for a wide variety of applications. Typically, a movable platform will be supported for sliding movement with respect to a stationary member or base. A bearing system including, for example, ball bearings or cross roller bearings provide for sliding support of the movable member with respect to the stationary member or base. Such arrangements can provide linear movement, or compound movement, for example by providing a pair of superposed sliding assemblies with respective movements which are orthogonal to one another. Such single or multi-directional arrangements can be utilized in lathes, drill presses, plotters, or even in seat adjustment mechanisms for a vehicle. Some applications require extremely precise and well controlled movement of the movable member or stage with respect to the stationary member or base. For example, when a movable platform is utilized for controlled movement of a sample which is being examined by a microscope, movement must be extremely precise and controlled, while avoiding undesired movement such as vibrations.
FIG. 1 shows a typical movable stage arrangement in which a stage 4 is supported upon a stage base 7 via ball bearing assembly 5, which includes ball bearings 6. The stage 4 is thus linearly movable with respect to the stage 7 in a direction into and out-of the drawing figure. In addition, controlled movement of the stage with respect to the base is provided by a leadscrew assembly shown generally at 8. The leadscrew is typically assembled upon the stationary stage base, with the leadscrew nut attached to the movable stage top. Rotation of the lead screw thereby controls relative movement between the stage 4 and the base 7. Further, as shown in FIG. 1, such an arrangement can be utilized for controlled movement of a sample 3 being examined by a probe microscope to allow the probe 2 of the microscope 1 to examine the sample 3.
In the context of a scanning probe microscope, the stage is utilized to provide linear travel by moving the stage top to successive new locations, with the stage top hopefully maintaining a fixed position at each stopping point. The sample or specimen may then be inspected, for example by a scanning probe microscope 1 which is mounted above the stage. Typically, bearing arrangements which provide the relative movement between the stage and base are stiff in all possible axes of motion, except for the axis of travel of the stage. However, in the context of an extremely sensitive microscope, even the slightest of vibrations can impose severe difficulties in obtaining satisfactory resolution of the sample being examined. Often, even vibrations from a spoken voice can severely deteriorate the information obtained by the microscope.
Providing high stiffness in holding the stage top in place is thus extremely important. In the case of a scanning probe microscope, the microscope can have a resolution of less than one angstrom. Therefore, the sample and stage must be held rigidly in place such that any vibrations or movement are a small fraction of an angstrom. However, while the prevention of inadvertent movement is essential, any technique utilized for fixing a position should not impede the intended motion of the stage.
FIG. 2 shows one attempt to maintain a fixed rest position of a movable stage, wherein like reference numerals designate corresponding parts to the stage shown in FIG. 1. In the FIG. 2 arrangement, a clamp device 9 is provided at a location adjacent to the movable stage 4. When the stage is moved to a desired rest position, the clamp device 9 is activated such that it engages the movable stage 4 and thus holds the top rigidly in place. The clamp is then moved out of engagement for movement to a next rest position. Such an arrangement suffers from a number of shortcomings. For example, when the clamp is activated, new forces are introduced into the system, and particularly upon the stage. As a result, the stage and/or sample mounted thereon can move, making it extremely difficult to achieve and maintain a precisely desired rest position. The bearings have a finite stiffness, and the new forces introduced by the clamp typically cause microns of undesired motion of the stage top.
In addition, a clamp arrangement also complicates the system, since it requires various components associated with control of the clamp. Typically, the stages are automatically controlled, and the electrical power of an on/off relay must be provided, as well as an automatically controlled coordination of the clamping with the movement of the stage. In addition, the clamp is often piezoelectrically activated, and as such, can require high voltages often between 100-1000 volts. Electrical power utilized with the clamp can also introduce heat into the system which is particularly undesirable in extremely precise operations. In the case of solenoid actuation of the clamp, the amount of undesirable heat generated by the solenoid can be significant.
A further disadvantage of clamp techniques relates to the time required to activate the clamp. Often movable stages are utilized in high throughput operations, and the additional time required to activate the clamp (1) increases the time required for each movement; and (2) does not decrease the "settling time" of the stage. The settling time refers to the amount of time for required oscillations of the stage to cease after the stage has moved to a new rest position.
Clamping arrangement can also be unreliable as a result of the numerous additional components introduced into the movable stage system. In addition, in the case of a piezoelectrically activated clamp, only limited travel of the clamp is obtainable, and therefore particular care must be taken in machining and assembling the stage such that the surface of the movable stage does not move out of the range of the clamp as the stage moves along its length of travel. The clamping surface of the stage must be extremely parallel to the travel axis of the stage.
As an alternative to automatic clamps, manually activated clamps may be utilized, however manual clamping is not practical for automated operations, particularly high output operations. In addition, manual clamps also retain the disadvantageous introduction of transient forces into the system.
In addition, if movement prevention is provided by merely utilizing an extremely stiff leadscrew and nut, as well as the bearings holding the leadscrew, the expense of the stage assembly can increase dramatically, while still yielding less than satisfactory results.
Accordingly, an improved movable stage assembly is desired in which the stage can be successively moved to plural rest positions, with the stage securely positioned at each of the rest positions such that undesired movement (e.g., vibration) at the rest positions is reduced. The movable stage should be capable of moving from one rest position to another, without having the movement impeded by any devices which prevent movement at the rest position. In addition, a device or assembly which prevents undesirable movement of the stage should be inexpensive, preferably requiring no additional power sources or automatic control devices.